The utilization of plastic and thermoplastic substrates in various industries typically requires the use of curable adhesive compositions to affix or bond the substrate to another structural component. The bond formed between the plastic or thermoplastic substrate and the structural component must meet certain requirements of adhesive strength depending on the particular application. One example of an industrial utilization of plastic substrates involves the use of rigid fiber-reinforced plastic composite materials in the form of sheet molding compound (SMC) in the automobile industry as an alternative to steel automotive body panels in an effort to reduce weight and corrosion susceptibility of an automobile, van, truck or the like. Sheet molding compound is typically comprised of various resin compositions such as a polyester resin reinforced with, for example, glass fibers. The sheet molding compound is molded under heat and pressure in order to prepare a rigid, self-supporting, fiber-reinforced structure.
One example of an adhesive composition which has previously been described as being useful for bonding sheet molding compounds and other substrates is disclosed in U.S. Pat. No. 4,578,424. The adhesive is a two-component adhesive wherein the first component of the adhesive contains an epoxy resin and an additive selected from the group consisting of a polyisocyanate, a carboxylic anhydride, and molecules with unsaturated carbon-carbon bonds capable of undergoing Michael addition reaction with amines. The second component of the adhesive is a hardener component for curing the first component and contains a mixture of amido amines, primary and secondary amines having tertiary amine groups or ether groups in their backbone, and bis-phenol A. Maleic or fumeric groups are given as examples of molecules with unsaturated carbon-carbon bonds capable of undergoing Michael addition reaction with amines.
Another adhesive composition previously described in the patent literature as being useful for bonding sheet molding compounds and other substrates is described in U.S. Pat. No. 4,803,232. This adhesive composition is a two-component system wherein the first component contains an epoxy resin and an acrylate or methacrylate ester of an aliphatic polyhydric alcohol. The second component of the adhesive composition contains an amine-terminal butadiene-acrylonitrile rubber, at least one aliphatic or aromatic polyamine, and at least one polyamido-amine.
Previously developed structural adhesive compositions such as those described above are more effective when the surface of the substrate to be bonded is specially prepared prior to application and curing of the adhesive. Typical surface preparation techniques involve solvent wiping, abrading or priming, all of which can be cumbersome, expensive and time-consuming. Another disadvantage associated with many traditional structural adhesive compositions is that they often require the addition of a rubber component, such as a carboxylic acid group- or amine group-terminated butadiene/acrylonitrile copolymer rubber, to the epoxy resin in order to impart needed flexibility to the resulting bond. Furthermore, many structural adhesives lack versatility in that they can only be utilized to bond a specific type of substrate.
A need therefore exists for a structural adhesive composition that can be applied to a variety of unprepared substrates so as to produce a flexible adhesive bond without the need for an additional rubber component. It would also be desirable for such an adhesive composition to cure at a reasonably rapid rate and to produce a bond which exhibits a high degree of strength.